The present invention relates in general to measurement of crankshaft acceleration in an internal combustion engine corresponding to each of a plurality of engine cylinder power strokes, and more specifically to the selection of a particular interval occurring once per power stroke for measuring a velocity to be used in calculating the accelerations.
In the typical four-stroke internal combustion engine, the four strokes include the intake stroke, the compression stroke, the power stroke, and the exhaust stroke One engine cycle is comprised of 720.degree. of crankshaft rotation during which each cylinder passes through each of its four strokes. In any engine having more than four cylinders, the power strokes of different cylinders overlap.
Gas pressure from combustion in a properly firing cylinder accelerates the engine crankshaft during the power stroke of that particular cylinder If a cylinder misfires instead of properly firing, then friction, compression in other cylinders, and external load combine to produce a net deceleration during the power stroke. By virtue of its mass and the arrangement of cylinders, an engine is intentionally designed to provide smooth rotation. Therefore, the velocity changes resulting from the accelerations and decelerations is small compared to total rotational velocity The decline in rotational velocity during a single completely misfiring power stroke is less than 3% and typically is only a fraction of 1%. The overlapping of power strokes in engines having more than four cylinders further masks the velocity effect of a misfiring cylinder.
Nevertheless, there have been many attempts at performing engine diagnosis by measuring velocity fluctuations of the engine crankshaft. Specifically, measurements of velocity have been used to characterize engine acceleration, torque, and/or cylinder power contribution. The use of velocity measurements to obtain such engine parameters has the advantage of being noninvasive into the engine itself. However, none of the prior attempts have had the sensitivity or reliability to allow accurate diagnosis of individual power strokes.
Most prior techniques have attempted to examine velocity fluctuations within the power stroke, such as by measuring instantaneous crankshaft velocity at predetermined points within a power stroke. For example, Rackliffe et al, U.S. Pat. No. 4,064,747, teaches a system wherein two or more measurements of instantaneous, subcyclic speed within each power stroke are taken. The measurements are separated by a significant portion of the stroke. The difference in instantaneous speed is a measure of acceleration during that stroke. The relative acceleration between different power strokes gives the power contribution for a single power stroke.
Buck et al, U.S. Pat. No. 4,295,363, likewise teaches a system wherein a plurality of time intervals in each power stroke are measured to determine instantaneous, subcyclic speed. Buck et al describes a compression test conducted while cranking an engine without a fuel supply, and a low power test conducted only during a predetermined engine acceleration.
Citron et al, U.S. Pat. No. 4,532,592, determines an acceleration for a power stroke of a particular cylinder by measuring an instantaneous speed at the cylinder's top dead center (TDC) and at the top dead center of the next cylinder in the firing order. The difference of the instantaneous speed at the successive stop dead centers provides the acceleration for the particular power stroke. Citron et al, U.S. Pat. No. 4,697,561, discloses a modification where the instantaneous speeds for forming an acceleration measurement are the lowest speed point and the highest speed point within the velocity fluctuation of the power stroke.
The attempts to examine an instantaneous velocity during the fluctuations occurring within a power stroke have proven to be undesirable for several reasons. A principal disadvantage with such instantaneous velocity measurements is the presence of large errors in the measurements since the ratio of the position tolerance of the mechanical position indicators to the rotational angles being measured is large. In order to minimize errors, special position indicating mechanisms are required that have extremely high resolution and are formed according to exact tolerances to provide highly accurate position references.
When the velocity measuring interval (i.e., rotation angle) is small in order to closely approximate instantaneous speed, the amount of noise in the measurements is large. Such a system is subject to uncontrollable (i.e., nonsystematic) position errors, such as are caused by gear lash and twisting of the crankshaft. Thus, it is difficult and expensive to provide power stroke diagnosis based on measurements of instantaneous velocity within the velocity fluctuations.
Another attempt at characterizing engine performance based on velocity differences employed one longer angle velocity measurement once per power stroke. As disclosed in James et al, "Microprocessor Based Data Acquisition For Analysis Of Engine Performance", SAE Technical Paper Series, No. 870386, February 1987, the average angular acceleration between cylinder firings can be used to detect individual cylinder misfires, provided very precise measurements are made of engine rotation. Engine velocity is measured over one profile ignition pulse (PIP) interval which occurs once per cylinder firing (e.g., once every 120.degree. of crankshaft rotation in a six-cyinder engine). The PIP signal is a digital signal having a rising edge occurring at or just prior to top dead center of each respective engine cylinder. Although a misfiring cylinder does cause a dip in the acceleration measurements obtained using engine speed over each PIP interval, a single misfire may affect acceleration measurements corresponding to several consecutive firings so that it is not possible to identify the exact cylinder which misfired or the number of misfiring cylinders. Thus, accurate characterization of engine performance is not achieved.
Hanson et al, U.S. Pat. No. 3,972,230, detects misfires using crankshaft acceleration based on velocities measured using one pulse per power period. The velocity measurement periods are based on a tachometer signal and thus extend from one spark instant to the next. The resulting velocity intervals approximately correspond to an interval from top dead center to top dead center, but are necessarily somewhat variable. The lack of any fixed phase synchronization of the velocity intervals to crankshaft position and the approximate location of the intervals between top dead centers results in generally very high noise levels and in cross-coupling of misfire indications to adjacent power strokes.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for deriving an acceleration measurement for each power stroke requiring just one velocity measurement per power stroke while avoiding the need to measure instantaneous speed at any point within the power stroke.
It is a further object of the present invention to obtain an acceleration measurement having a maximum contribution from a respective cylinder's power stroke and a minimum contribution from all other power strokes even in engines having overlapping power strokes.
It is further object of the invention to obtain velocity and acceleration data for an internal combustion engine in which a high signal-to-noise ratio is achieved.